Those sources confirmed the weapon in question was purchased in South Florida quite some time ago. The last person registered as the owner in state and federal databases, is believed to be deceased.

So how did the gun, with 10 extra rounds loaded, end up wrapped in a towel inside a restroom garbage can? Was it a test of security measures to see if it could be found, laying the groundwork for a future incident? Was it part of a plot accidentally intercepted by an airport janitor before it was executed?

Those are some of the questions law enforcement want to answer. In the meantime, Monday’s discovery of the gun and ammunition has directly lead to an increase of airport security and presence of state and federal law enforcement officers in the terminal.

“This airport is a major target for people who want to do harm. We’re doing everything we can in the aftermath of this incident to keep our passengers and employees safe,” said Ret. Col. Emilio Gonzalez, who oversees MIA as Miami-Dade County’s Aviation Director.

In addition, the FBI confirmed agents serving on the Joint Terrorism Task Force are assisting in the investigation to determine who left the pistol grip shotgun in the bathroom by the Concourse D security checkpoint.

It was discovered by a custodian Rudolphe Guerrier who was cleaning the bathroom.

“As I went to change the garbage can, I looked inside and saw the object in the can and then I called police,” said Guerrier. “It was wrapped in a towel. Then I put the plastic back around the garbage can.”

Guerrier was stunned he found a weapon inside the airport.

“I have never seen that before,” he said. “Never.”

Was it something as simple as a passenger realizing they couldn’t take their shotgun on a flight or was it left there for planned use by someone not yet known to investigators?

Airport Director Gonzalez said that’s highly unlikely.

“People don’t just show up at the airport with a loaded gun and extra ammo by accident,” he said.

Those close to the investigation said Miami-Dade detectives are said to be doing a lot of legwork on this case. Those detectives are getting help from the Department of Homeland Security, Customs and Border Protection, and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The incident comes at a time of heightened alert. The FBI and Homeland Security are warning that ISIS-inspired Americans or visitors may try to launch a domestic attack in retaliation for airstrikes in Syria. Senior airport officials said they are also troubled by reports an ISIS fighter killed in combat in Syria once had security clearance as a fuel truck operator at the Minneapolis-St. Paul Airport.

There is also concern over Khorosan, an off-shoot of Al Qaeda, described by terrorism experts as worse than ISIS. Khorosan threats against international aviation led to recent changes in security procedures for all US-bound international travelers who must power up their phones and other electronics before clearing security in foreign countries. These measures were put in place recently when U.S. Intelligence uncovered information Khorosan was trying to develop explosives disguised as batteries that were not detectable by conventional means.